This invention relates generally to improvements in color television receivers. It is particularly directed to systems which comb-filter luminance components from color components of a television signal.
In NTSC type broadcast signals, luminance and color components are frequency-interleaved. At the receiver, those components are separated from each other and processed separately to develop a video image.
To effect the required separation between the luminance and color components, various well known types of filters may be employed. One such filter is known as a comb filter in which the combined video signal is processed along two parallel paths, one path having an 1-H (one horizontal line interval) delay greater than the delay associated with the other path. The signals which are output from each path are combined so that color components are separated from luminance components. For good performance, the delay and gain associated with each path must be stable and accurate.
Since the commercial arrival of CCD's (charge coupled devices) it has been proposed to use CCD's for constructing comb filter delay elements. One CCD provides the delay associated with one processing path, and another, shorter CCD provides the delay associated with the other path. U.S. Pat. No. 4,096,516 describes such a CCD comb filter.
In the system described in the above-mentioned patent, the CCD delay lines are clocked at a frequency of three times the color subcarrier frequency, and an inverter is included in one delay path for proper combing operation. It is thought that the need for such an inverter can contribute to a difference in the required 1-H delay between the two paths and also contribute to an undesired difference in gain between paths. Hence, some kind of gain adjustment and/or delay adjustment is thought to be necessary for satisfactory commercial implementation of this system. In addition, the above-mentioned CCD comb filter system requires, of course, one or more color demodulators for developing baseband color signals from the separated color components.
For application of CCD comb filters to high volume commercial television receivers, it is preferable that no adjustment be required of the gain or delays associated with the filter. In addition, power can be saved if the frequency of the CCD's clock input is lowered. A further, and very significant, improvement in such comb filters could be realized if the CCD's which are employed as a comb filter were also capable of operating as a color demodulator.